THE BROOKS RANGE, ALASKA: THE CONSEQUENCE OF FOUR DISTINCT TECTONIC EVENTS

The Brooks Range is generally considered to be an orogen that had a protracted history of deformation extending from the Jurassic to the Neogene. However, field studies, seismic reflection data, Ar-Ar dating, and fission-track data support the idea that the Brooks Range was constructed instead as a consequence of at least four unrelated tectonic events that have distinctive timing, structural style, levels of detachment, structural relief, and tectonic origin. The earliest event was a north-directed arc-continent collision that was active from ~145-112 Ma. This event involved a thin (1-4 km thick) stratigraphic section, producing a thin-skinned deformational wedge characterized by far-traveled allochthons in the foreland and by blueschist-facies metamorphism in the hinterland. The second event was crustal-scale tectonic extension that occurred in the hinterland of the first deformation during the mid-Cretaceous (~103-96 Ma). This event thinned previously thickened crust and produced regional extensional unroofing that reset Ar-Ar ages and shed voluminous clastic sediments into the foredeep of the older collisional orogen and into depocenters in the extended region. Following a period of reduced tectonic activity, north-directed thrusting resumed in the early Tertiary (~60-45 Ma). This event involved basement in the hinterland and a structural section 5-10 km thick in the foreland, and terminated in a thin-skinned frontal triangle zone. This thrust belt is characterized by a relatively low amount of shortening, but high structural relief that reset apatite fission-track ages. The final deformational event was areally restricted to the eastern Brooks Range and included episodic north-directed contraction active at 35 Ma, 25 Ma, and at the present. This event produced a structural salient marked by basement-involved deformation onshore and an active thin-skinned frontal zone offshore.

The Jurassic to Cretaceous collisional event, caused by south-directed subduction, was replaced by north-directed subduction in southern Alaska at the time of the mid-Cretaceous extensional event. Tertiary events are the result of retroarc thrusting that emanated from subduction in southern Alaska, followed by collision of the Yakutat terrane with the southern Alaska continental margin in the Neogene.